The canonical Wnt signaling pathway is involved in various differentiation events during embryonic development and when aberrantly activated, can lead to tumor formation. Our long term goal is to understand how Wnt co-receptor LRP6 expression and function is regulated in cancer cells and how changes in its expression and function contribute to tumorigenesis so that effective therapeutics can be developed. Wnts bind to their receptors LRP6 and frizzled protein (Fz), and initiate a cascade of intracellular events to activate the Wnt signaling. The secreted Wnt antagonist, Dickkopf (Dkk), interferes with Wnt signaling in vertebrates by binding directly to LRP6. Furthermore, Dkk interacts with another transmembrane protein, Kremen. The LRP6-DKK-Kremen complex is internalized, thus removing LRP6 from the cell surface. In our preliminary studies, we found that LRP6 expression is involved in breast cancer Wnt signaling and mammary development. More importantly, our preliminary studies have demonstrated that Dkk1 alone is able to stabilize LRP6 while antagonizing Wnt signaling, revealing a novel role for DKK1 on Wnt ligand-induced LRP6 down-regulation and Wnt signaling. We hypothesize that while Wnt binding induces LRP6-Wnt-Fz complex formation and Wnt signaling activation, the endocytosis and desensitization of this complex takes place after Wnt signaling activation. By preventing the Wnt binding to LRP6, DKK1 alone (without Kremen) can prevent LRP6-Wnt-Fz complex formation and internalization, and stabilize LRP6 on the cell surface at a non functional status. We further hypothesize that there are two LRP6 turnover pathways: Kremen-mediated LRP6 degradation and Wnt/Fz-mediated LRP6 down-regulation. The former removes LRP6 from the cell surface and makes LRP6 unavailable for Wnt signaling, and the latter desensitizes Wnt signaling after ligand binding. The objective of this project will be accomplished through three specific aims: (1) to dissect molecular and cellular mechanisms underlying Kremen-mediated LRP6 turnover, (2) to dissect molecular and cellular mechanisms underlying Wnt ligand-induced LRP6 down-regulation, and (3) to define the roles of LRP6 expression and turnover on Wnt signaling and mammary tumorigenesis. PUBLIC HEALTH RELEVANCE: The canonical Wnt signaling pathway is involved in various differentiation events during embryonic development and when aberrantly activated, can lead to tumor formation. The low density lipoprotein receptor-related protein 6 (LRP6) is critical for the activation of the Wnt signaling pathway. The goals of this project are to dissect molecular and cellular mechanisms underlying LRP6 turnover, and to define the roles of LRP6 expression and turnover on Wnt signaling and mammary tumorigenesis. The research proposal may lead to a novel strategy for cancer treatment.